OS 10.5 - Is it worth upgrading from Tiger to Leopard?

I bought my MacBook in March 2007. So far, I have no problems with it. That being said, my housemate just got a new Mac and asked if I wanted to use it to upgrade my operating system with his Leopard discs.

Two questions:
1 - Is that legal? More importantly, do I lose my warranty for doing that? Am i allowed to use his discs?

2 - Is it worth it? Do people think Leopard is enough of an upgrade that it's worth doing that?